A new biomimetic model of free radical reactivity in lipids

The modeling of free radical reactions under naturally occurring conditions has become a basic step in the research of fundamental mechanisms of biological processes. The far most known process regarding polyunsaturated fatty acids (PUFA) is the peroxidation process which is generally considered to be the major mechanism of cell injury in organisms subject to oxidative stress. A much recent research deals with the geometrical isomerization of PUFA, evidencing that the cis double bond configuration is a inherent characteristic of membrane lipids which can be altered by radical stress. In view of the fact that these processes in lipids are the most relevant chemical processes occurring to PUFA, it is very useful to perform model studies where both processes occur. We have established the simplest biomimetic model of cell membrane, the micelles of linoleic acid (LH), prepared by addition of a non-ionic surfactant (TWEEN®-20) and the resulting solutions were irradiated by ionizing radiation up to 400 Gy under a variety of conditions where thiyl radicals are the main reactive species. The irradiation-induced peroxidation and trans isomerization in our model systems under controlled oxidation conditions have been studied. The concentrations of hydroperoxide of linoleic acid (LOOH) were determined using the spectrophotometric ferric thiocyanate method, while geometrical isomers were analyzed by GC using known conditions for the separation of cis and trans isomers. Data on relative importance of these processes in this competitive environment have been considered. This lecture will demonstrate that hydroperoxy- and trans-PUFA can be the resulting effect of oxidative free radical conditions. While under anaerobic conditions only the cis-trans isomerization was observed, in air-equilibrated solutions a substantial amount of LOOH was produced and the cis-trans isomerization process was still observed, e.g., irradiation of 500 M of LH at 400 Gy and dose rate of 4.6 Gy/s gave 20 M of LOOH and 10% conversion of LH into mono-trans isomers. The effect of micelle size will be discussed in order to gather data on the effect of supramolecular organization for the outcome of the two processes, and in particular, to envisage any positional preference of the two double bond.